Fluid catalytic cracking (FCC) is primarily used to convert high boiling, high molecular weight hydrocarbons into lower boiling, lower molecular weight compounds. The lower molecular weight compounds include gasoline, olefinic compounds, liquid petroleum gas (LPG), diesel fuel, etc. An FCC unit typically uses a catalyst that is repeatedly deactivated and regenerated in a riser and a catalyst regenerator. Air is used to combust (burn off) coke from the deactivated catalyst in the regeneration process, and produces combustion gases such as carbon dioxide and water. Many FCC units use the energy generated from burning the coke from the catalyst to drive the endothermic reaction in the riser.
In a typical FCC units, a hydrocarbon feed stream is contacted with the catalyst at reaction conditions in the riser, and a layer of coke is deposited on the catalyst as the hydrocarbons are cracked into smaller molecules. The deposited coke shields the deactivated catalyst (often referred to as spent catalyst), and the coke is oxidized in the catalyst regenerator so the catalyst can be used again. In many FCC units, the catalyst feed to the catalyst regenerator includes some spent catalyst from the riser combined with some recovered catalyst that has passed through the catalyst regenerator. The spent catalyst produces more heat and combustion gases than the recovered catalyst because it has more coke, so the recovered catalyst is combined with the spent catalyst to better control the temperature in the catalyst regenerator. The spent catalyst and the recovered catalyst are typically combined near the bottom of a combustion chamber of the catalyst regenerator, and the two different catalyst streams normally do not mix well during the prescribed residence time. When the two streams of catalyst are fed into the chamber, they tend to rise straight up or otherwise move side by side without mixing so the spent catalyst distribution in the combustor remains uneven. The side of the combustor that has the spent catalyst tends to operate hotter than the side with the recovered catalyst, as well as producing more combustion gases. The uneven coke loading results in nonuniform combustion, although most combustors are designed for consistent operating conditions. The uneven heat and gas flow rates in the combustor can result in some carbon monoxide or other combustible gas rising into a catalyst separator of the catalyst regenerator, where it can further oxidize and produce undesired heat.
Accordingly, it is desirable to provide catalyst regenerators and methods for regenerating catalyst that mix spent catalyst with recovered catalyst before the mixed catalyst is introduced into the combustor. In addition, it is desirable to provide catalyst regenerators and methods for regenerating catalysts that reduce combustible gases in the catalyst separator of the catalyst regenerator. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawing and this background.